fill, stuff, full
牣 centers on the idea of being filled to capacity: stuffing something full or being completely full.
牣 combines 牜 (cow/ox) with 刃 (blade), but the historical origin is uncertain. It may have originally expressed filling or stuffing in a context related to livestock, but the exact development is unclear.
Imagine a cow (牜) being stuffed so full that its sides are as tight as a blade (刃). The cow is completely filled.
For ジン, picture a gin bottle being filled to the brim: gin -> ジン, and the bottle is stuffed full.